The Principles of Stratigraphical Geology

by J. E. Marr

Some further remarks will be made in subsequent paragraphs concerning the period of the earth's history at which the geologist is first furnished with definite records, but in the meantime it may be observed that the geologist will do well, when working amongst the strata, to consider that the more active operation of agents, even in times of which he has definite knowledge, may have produced effects which he should be prepared to discover, as their discovery would be of considerable importance, and that he should not be content to infer that because it has been proved that agents operating with the same intensity as that which they have at present, may have produced all the effects which he can actually observe, they therefore necessarily did produce them.

Recurrences. Absolute uniformity of conditions is impossible, even in a single area. Every change which takes place upon the earth produces conditions somewhat dissimilar from those which previously existed, and these will leave their effects upon the physiography of the area. For this reason, assuming that the conditions have gradually changed from simpler to more complex, every period of time will have been marked by conditions which never prevailed before or afterwards, and these will leave their impress upon the deposits of the period. It is doubtful for instance, as already remarked, whether the exact conditions which gave rise to the extensive deposits of vegetable matter in Carboniferous times which now form coal, ever occurred to a like extent in previous or subsequent periods, and accordingly, though we have deposits of coal of other ages, none are so extensive as those of the Coal Measures. Again, as the strata of one period are largely composed of denuded particles of pre-existing strata, which were derived directly or indirectly from igneous rock, the soluble material existing in the igneous rocks must have been gradually eliminated unless restored by other processes, and we might expect to find that early sediments have, on the whole, a larger proportion of soluble silicates than the later ones.

Besides these changes, there are physical changes which are recurrent, and cause conditions generally similar to pre-existing ones to occur in an area after an interval of dissimilar ones. We have seen that deposits tend to vary according to the distance from the coast, limestone being succeeded by mud, this by sand and gravel, and after subsidence the sand and gravel are succeeded by mud, and that by limestone. These changes will produce some effect upon the organisms, and the recurrence of organisms is a well-known event, of which cases have been cited in a former chapter.

Again we find, as already pointed out, recurrence of climatic changes, with alternation of glacial and warmer periods, and these may have been very widespread, and would influence the other physical conditions, as well as the distribution of the organisms. Vulcanicity may have been more rife at some periods than others, for instance there seems, in the present imperfect state of our knowledge, evidence of enfeebled vulcanicity in later Mesozoic times, and of its renewed activity in Tertiary times. Again, orogenic movements seem to have occurred more extensively at some times than others, as for instance in early upper Palæozoic times, at the end of the Palæozoic epoch, and in early Tertiary times, though this may also be an apparent and not an actual truth, due to imperfect knowledge. In any case, in limited areas, there seem to have been alternations of periods of uplift accompanied by marked orogenic movements, and of widespread depression, accompanied by sedimentation.

The subject of rhythmic recurrence is worthy of further study. This recurrence in combination with evolutionary change may account for the apparent marked difference between Cambrian and Precambrian times, a difference which strikes some geologists as being too great to be accounted for as due to our ignorance only.

Organic evolution. This subject is too wide for more than passing notice in a work of this character. The evidence of Palæontology is of extreme importance to the biologist, and indeed, the way in which evolution of organisms has occurred can only be actually demonstrated by reference to Palæontology, and the study of Palæontology has already given much information concerning the lines on which evolution has proceeded in different groups of organisms. It must be remembered that the major divisions of the invertebrata were in existence in very early times; indeed representatives of most of them are found in the rocks containing the earliest known fauna, that of the Olenellus beds of Cambrian age. If our present views as to evolution be correct, there is no doubt that the period which elapsed between the appearance of life upon the globe and the existence of the Olenellus fauna must have been very great, possibly, as Huxley suggested, much greater than that which has elapsed between early Cambrian times and the present day. If this be so, however probable it is that we shall carry our knowledge of ancient faunas far back beyond Cambrian times, it is extremely improbable that we shall ever get traces of the very earliest faunas which occupied our earth.

Geological time. Various attempts have been made to give numerical estimates of the lapse of time which occurred since the earth was formed, or since the earliest known rocks were deposited. These attempts may be classed under two heads, namely, those made by physicists, mainly on evidence obtained otherwise than by a study of the rocks, and those made by geologists by calculating the mean rate of denudation and deposition of the rocks, and estimating the average thickness of the rocks of the geological column.

The calculations of physicists as to the age of the earth vary:—Lord Kelvin assigned 20,000,000 years as the minimum and 100,000,000 as the maximum duration of geological time. Prof. Tait has halved Lord Kelvin's minimum period, while Prof. G. Darwin admits the possibility of the lapse of 500,000,000 years.

The estimates made by geologists, which will appeal more directly to the geological student, also vary considerably, though they bear some proportion to those which have been put forward by the physicists. Prof. S. Haughton[128] assigned a period of 200,000,000 years for the accumulation of the rocks of the geological column; Mr Clifton Ward[129] one of 62,000,000 years, after studying the rocks of the English Lake District, and allowing for the gaps in the succession; Mr A. R. Wallace[130] further lowers the time for the formation of the column to 28,000,000 years; Sir A. Geikie[131] gives 73,000,000 years as the minimum and 680,000,000 as the maximum; while Mr J. G. Goodchild has lately[132] estimated the period at over 700,000,000 years.